This invention relates to resin-based dental restoratives, and more specifically to restorative compositions that exhibit high condensability, low volumetric shrinkage and improved wear/abrasion resistance.
Posterior and anterior tooth restoration is typically accomplished by excavating a tooth that has decayed or is otherwise in need of repair to form a cavity. This cavity is filled with a paste material, which is then compacted and shaped to conform to the original contour of the tooth. The paste is then hardened, typically by exposure to actinic light. The paste material is a tooth colored, packable, light curable, polymerizable restorative composition comprising a highly filled material.
Posterior tooth restorations, especially the Class II type, require the use of a matrix band for proper application of a restorative. The restorative has to be condensable. That is, as it is packed into the cavity of a tooth surrounded by a matrix band, the restorative must deform the matrix band in such a way that the original tooth contour is achieved. In addition, proper deformation of the matrix band leads to appropriate contact with the adjacent teeth.
Up to now, the only type of restorative having adequate rheological properties for use with a matrix band has been amalgam. Amalgams have been employed as restoratives for this purpose for a long time and they are known to have good wear characteristics, good marginal quality over time due to buildup of corrosion products at the border of the restoration and a small coefficient of thermal expansion. The metallic color, however, is a drawback for their use as is the uncertainty of the biological interactions of the metallic components of dental amalgams.
Tooth colored dental restorative composites are usually composed of dispersions of glass filler particles below 50 xcexcm in methacrylate-type monomer resin. Splintered pre-polymerized particles, which are ground suspensions of silica in pre-polymerized dental resins, may also be used. Additives such as pigments, initiators and stabilizers have also been used in these types of composites. Because the glass particle surface is generally hydrophilic, and because it is necessary to make it compatible with the resin for mixing, the glass filler is treated with a silane to render its surface hydrophobic. The silane-treated filler is then mixed with the resin at a proportion (load) to give a paste with a consistency considered usable, that is to allow the paste to be shaped without it flowing under its own weight during typical use. This paste is then placed on the tooth to be restored, shaped and cured to a hardened mass by chemical or photochemical initiation of polymerization. After curing, the mass has properties close to the structure of a tooth.
Although it has been found that increasing the load of a resin-based composite leads to higher viscosity, amalgam-like condensability has not yet been achieved. There is thus a need in the dental profession to have a resin-based restorative that is condensable and compatible with the use of a matrix band.
As stated previously, the resins typically used in dental restorative compositions are mostly comprised of dimethacrylate monomers. These monomers vitrify quickly upon initiation of polymerization by crosslinking. The added glass particles after polymerization give a higher modulus to the system and reduce crack propagation by dispersion reinforcement.
A significant disadvantage in the use of methacrylate resin-based restorative composites is that they shrink significantly after cure. For example, a modern hybrid composite shrinks approximately 3% after cure. This shrinkage leads to further tooth decay because bacterial infiltration is possible. To address the problem of tooth decay, adhesives are used to coat the tooth surface to be restored before the application of the composite. The shrinkage stress during the initial phase of the vitrification process, however, is significant and on the order of 1 MPa or higher during the first 20 seconds of light exposure for a light cure composite. This initial stress development compromises the performance of the adhesive. So even with the use of an adhesive, significant marginal breakdown can occur, leading to bacterial infiltration. This process is defined as microleakage and is usually measured by dye penetration methods. Thus, there is also a need to make available to the dental profession a resin-based composite that has reduced volumetric shrinkage and shrinkage stress.
The coefficient of thermal expansion of the glass fillers used in resin-based composites is much closer to tooth structure than that of the resins. So it is desirable to limit the amount of the resin in a dental composite and maximize the amount of filler material. The main factor limiting the volume fraction (load) of the inorganic filler in highly filled suspensions is particle-particle interactions. Dispersants, through their ability to reduce interactions between particles can improve the flow (reduce the viscosity) of the suspension, therefore allowing a higher load. Dispersants in non-aqueous systems reduce particle interactions by a steric stabilization mechanism. A layer of the dispersant is adsorbed on the surface of the particles keeping them apart from one another, reducing the viscosity. The dispersant structure must contain a chain that allows for steric stabilization in the resin and it also must be strongly adsorbed on the particle surface. There is thus a further need to provide a dispersant that will be effective with a non-aqueous, highly filled suspension containing polymerizable groups for use in a dental restoration.
An additional critical area needing improvement in dental restorations is the wear and abrasion resistance of polymeric restorative compositions. For posterior restorations, the main wear mechanism is generally classified as the three body type, involving food bolus. For anterior restorations, wear is generally classified as the two body type, involving toothbrush abrasion, for example. Wear is caused by the heterogeneous nature of dental composites, occurring mostly through xe2x80x9cpluckingxe2x80x9d of the filler particles from the surface followed by abrasion of the softer resin phase. Because wear in these systems is highly dependant on friction, friction reducing additives are expected to improve abrasion resistance. For example, in Temin U.S. Pat. No. 4,197,234, polytetrafluoroethylene powder or another similar polyfluorocarbon resin or polyfluorochlorocarbon resin is added for improvement of abrasion resistance in a chemically cured dental composite. The polytetrafluoroethylene additive or other similar additives, however, also act as an opacifying agent, making the restoration nonaesthetic. In other words, the color of the restoration does not blend sufficiently with the surrounding dentition. In addition, when the opacity is high, light cure initiation cannot be used. Similarly, Fellman et al. U.S. Pat. No. 4,433,958 describes the use of several fluoropolymers as solid particulate insoluble in the liquid monomer system in dental restorative formulations. Again, highly opaque materials are obtained. There is thus an additional need to provide a dental restorative composite with superior wear and abrasion resistance in both posterior and anterior applications, without causing undue opacity in the restorative.
In summary, the dental profession is in need of a dental restorative that has improved shrinkage properties, higher load capabilities and superior wear and abrasion resistance, and that is condensable and compatible with the use of a matrix band.
The present invention provides a resin-based dental restorative that exhibits one or more of the following properties: high condensability, low volumetric shrinkage, low shrinkage stress, higher loading, lower coefficient of thermal expansion, and high wear and abrasion resistance. In its broadest form, the dental restorative composition of the present invention includes (1) a polymerizable (meth)acrylic monomer; (2) filler; and (3) one or more of the following additives: a rheological modifier in an amount effective to reduce the volumetric shrinkage of the dental restorative during polymerization/curing; a phosphate-based dispersant; and a fluorocopolymer that is soluble in (meth)acrylate resin.
Suitable rheological modifiers for use in the present invention include, among others, the following two types of compounds:
(1) a hydroxyfunctional polycarboxylic acid amide according to the formula 
wherein the symbols have the following meanings:
R=aliphatic hydrocarbon groups having 6 to 60 carbon atoms, or aromatic hydrocarbon groups having 6 to 20 carbon atoms, or aliphatic or aliphatic/aromatic hydrocarbon radicals having 6 to 150 carbon atoms which are interrupted by 2, 4, 6 or 8 carboxamide groups, or aliphatic hydrocarbon radicals having 4 to 150 carbon atoms which are interrupted by 2 to 75 xe2x80x94Oxe2x80x94 (oxygen) groups;
Rxe2x80x2=H, or C1, to C4 alkyl, or xe2x80x94Zxe2x80x2xe2x80x94(Q)yxe2x80x94(OH)x;
x=1 to 3;
y=0 or 1;
Z=an alkylene radical having 2 to 6 carbon atoms;
Zxe2x80x2=an alkylene radical which is identical to or different from Z, having 2 to 6 carbon atoms;
Q=an aliphatic hydrocarbon radical having 2 to 200 carbon atoms, which is linked via xe2x80x94Oxe2x80x94 or 
xe2x80x83to Z or Zxe2x80x2 and is interrupted by zero to 99 oxygen atoms and/or carboxylic acid ester groups; and
n=2 to 3; and
(2) the reaction product of:
(a) from about 15 to 75 parts by weight of one or more liquid polyalkoxylated nitrogen-containing compounds containing more than one hydroxyl group and which also contain a pendant aliphatic radical of 6 to 40 carbon atoms selected from the group consisting of tertiary amines and amides of secondary amines;
(b) from about 8 to 90 parts by weight of one or more polycarboxylic acids; and
(c) from about 0.5 to 20 parts by weight of one or more liquid diamines of a molecular weight of about 2000 or less,
wherein the reaction is continued until the acid value is within the range of 5 to 14 and the amine value is within the range of 42 to 84.
It has been found that the inclusion of either of the above rheological modifiers in the resin and filler composition of the present invention improves the condensability and shrinkage properties of the resulting composite. By way of example, but not limitation, if the first mentioned modifier is used it is preferably present in an amount of about 0.1 to about 0.7 weight percent, and if the second modifier is used it is preferably added in an amount of about 0.1 to about 1.5 weight percent of the total mixture.
Suitable phosphate-based dispersants for use in the present invention include, among others, the following types of compounds:
(1) a phosphoric acid ester according to the formula 
wherein n=5 to 10 and m=1 to 20; and
(2) a phosphoric acid ester according to the formula 
wherein R is a (meth)acrylate group functionalized radical.
R is preferably one of the following radicals: oxyethyl methacryloyl-, oxyethyl acryloyl-, polyoxypropyl methacryloyl-, glyceryl dimethacryloyl-, and dipentaerythritol pentaacryloyl-. The inclusion of either of the above types of dispersants or a combination thereof in the resin and filler composition of the present invention increases the filler loading, which results in reduced shrinkage, a lower coefficient of thermal expansion and generally improved physical properties. The dispersant is preferably present in an amount of 5 weight percent or less of the total mixture.
One suitable fluorocopolymer for use in the present invention is soluble in (meth)acrylate resins and is comprised of about 40-60 mole percent fluoroolefin units, about 5-45 mole percent cyclohexyl vinyl ether units, about 5-45 mole percent alkyl vinyl ether units and about 3-15 mole percent hydroxyalkyl vinyl ether units. The inclusion of this type of fluorocopolymer reduces the wear of the composite material. The fluorocopolymer is preferably present in an amount of 10 weight percent or less of the total mixture.
There is thus provided a dental restorative having improved thixotropic and physical properties and improved wear resistance. These and other objects and advantages of the present invention shall become more apparent from the description of the preferred embodiments and the examples.
In connection with the present invention, it has been discovered that a) the addition of a suitable rheological modifier to a (meth)acrylate resin-based restorative composite improves the condensability and shrinkage properties of the resulting composite, without negatively impacting other critical properties; b) the addition of a suitable dispersant of the phosphoric acid ester type increases the filler loading, and after curing provides a composite with reduced shrinkage characteristics; c) the addition of a suitable fluorocopolymer reduces the wear of the composite without negatively impacting physical and aesthetic properties; and d) the addition of a combination of two or more of a suitable rheological modifier, dispersant and fluorocopolymer provides a dental restorative composite with improved condensability, shrinkage, wear, filler load and other physical and aesthetic properties.
Dental Restorative Composite with Rheological Modifier
Ordinarily, the restoration of posterior teeth, in particular Class II restoration, involves one or more side surfaces in addition to the top surface of the tooth. After preparation of the cavity, a matrix band is placed. The matrix band is a thin, malleable metal or plastic sheet designed to fit around the side surfaces of the tooth and designed to be capable of being tightened. Tightening the matrix band results in intimate contact with said tooth surfaces. Manipulation of the matrix band with dental instruments may then be necessary to achieve the original tooth contour. The filling of the tooth is accomplished by an opening at the top surface. When the tooth is filled with amalgam, the amalgam is condensed (compacted) in such a way as to deform the matrix band further to give a better approximation of the original contour of the tooth. Heretofore, this type of deformation has not been possible with previously available resin-based composites even though they have been recommended for use in posterior tooth restoration. It is believed that only by using the materials described in the present invention is amalgam-like condensation possible. This is accomplished by the addition of a rheological modifier to the resin and filler mixture. While various rheological modifiers known for non-dental applications were tested for use in dental restoratives of the present invention, it was found that only certain such modifiers provide the desired properties of increased condensability, lower volumetric shrinkage and reduced shrinkage stress:
One such modifier is Formula (1): a hydroxyfunctional polycarboxylic acid amide according to the formula 
wherein the symbols have the following meanings:
R=aliphatic hydrocarbon groups having 6 to 60 carbon atoms, or aromatic hydrocarbon groups having 6 to 20 carbon atoms, or aliphatic or aliphatic/aromatic hydrocarbon radicals having 6 to 150 carbon atoms which are interrupted by 2, 4, 6 or 8 carboxamide groups, or aliphatic hydrocarbon radicals having 4 to 150 carbon atoms which are interrupted by 2 to 75 xe2x80x94Oxe2x80x94 (oxygen) groups;
Rxe2x80x2=H, or C1 to C4 alkyl, or xe2x80x94Zxe2x80x2xe2x80x94(Q)yxe2x80x94(OH)x;
x=1 to 3;
y=0 or 1;
Z=an alkylene radical having 2 to 6 carbon atoms;
Zxe2x80x2=an alkylene radical which is identical to or different from Z, having 2 to 6 carbon atoms;
Q=an aliphatic hydrocarbon radical having 2 to 200 carbon atoms, which is linked via xe2x80x94Oxe2x80x94 or 
xe2x80x83to Z or Zxe2x80x2 and is interrupted by zero to 99 oxygen atoms and/or carboxylic acid ester groups; and
n=2 to 3; and
Another such modifier is Formula (2): the reaction product of:
(a) from about 15 to 75 parts by weight of one or more liquid polyalkoxylated nitrogen-containing compounds containing more than one hydroxyl group and which also contain a pendant aliphatic radical of 6 to 40 carbon atoms selected from the group consisting of tertiary amines and amides of secondary amines;
(b) from about 8 to 90 parts by weight of one or more polycarboxylic acids; and
(c) from about 0.5 to 20 parts by weight of one or more liquid diamines of a molecular weight of about 2000 or less,
wherein the reaction is continued until the acid value is within the range of 5 to 14 and the amine value is within the range of 42 to 84.
It is believed that the Formula 1 modifier may be obtained from BYK Chemie USA, Wallingford, Conn., under the trade name BYK(copyright)-405. The Formula 2 modifier, it is believed, may be obtained from Rheox Corporation, Hightstown, N.J. under the trade name Thixatrol(copyright) VF-10. Either modifier has the effect of providing pseudoplastic and thixotropic properties to the composite pastes. These rheological modifiers and their thixotropic properties are described in U.S. Pat. Nos. 4,857,111 and 5,536,871, respectively, the entire disclosures of which are incorporated herein by reference. The condensable nature of the compositions containing either modifier, or both modifiers in combination, allows for the accomplishment of the contour without voids and gaps because the material offers resistance to packing. The condensable compositions of the present invention are also useful for those restorations not requiring a matrix band, such as Class I, III and V.
The rheological modifiers may be added directly during the mixing of the paste when the resin and the filler are combined in a planetary mixer. Alternatively, a solution of the rheological modifier in a volatile solvent, such as 10 percent modifier in ethanol, may be sprayed on the filler, followed by drying. This is the preferred method for formulating the rheological modifier into composites that are self-cured and powder-liquid. The modifier is added in an amount effective to achieve the desired properties of reduced volumetric shrinkage and shrinkage stress and improved condensability. This amount is variable depending on the compositions used for the resin and filler, but for example the range of 0.1 to 5 weight percent is contemplated. For the Formula 1 modifier the amount is likely to be in the range of about 0.1 to about 0.7 weight percent and about 0.1 to about 1.5 weight percent for the Formula 2 modifier. If too much modifier is added, the composite becomes too thick and will be difficult to manufacture and manipulate. If too little modifier is added, the desired effects will not be achieved. In a preferred embodiment of the present invention, 0.3 to 0.6 weight percent of Formula 1 modifier or 0.5 to 1.2 weight percent of Formula 2 modifier is added to the composite paste.
When the cavity to be filled is more than 2 mm deep, conventional light-cured resin-based composites must be layered with a layer thickness of 2 mm maximum in order to minimize the effects of the shrinkage occurring during polymerization. Because the compositions of the present invention show reduced shrinkage when cured and permit adequate depth of cure, the layering technique used during the placement of conventional light-cured resin-based composites can be eliminated or the layer thickness can be significantly increased, making placement simpler and less technique-sensitive when using the compositions of the present invention. The following examples will further illustrate the advantages of this aspect of the present invention.